1. Field of the Invention
The present invention generally relates to a hybrid electric vehicle, and more particularly to a generation control technique for a hybrid electric vehicle.
2. Description of Related Art
In recent years, a series hybrid vehicle equipped with a motor as a source of driving force for the vehicle and charges a secondary battery, which supplies power to the motor, by a generator driven by a relatively-small engine has been developed.
In this series hybrid vehicle, the power (rotational force) from driving wheels runs the motor to generate a braking force when the vehicle is braked. On the other hand, the power generated by the motor is utilized as regeneration energy to enable a so-called regenerative braking. The power acquired by the regenerative braking is normally charged in the battery.
If, however, the power acquired by the regenerative braking is in excess of the maximum charge level of the battery as in the case where the vehicle is suddenly braked, the regenerative braking should not be used to avoid over-charging the battery. It is therefore unavoidable to use a friction brake (service brake), and the use of the friction brake accelerates the wear of a friction material (i.e., brake pads). In order to ease the burden of the friction material, Japanese Patent Provisional Publication No. 7-131905 discloses a device, which supplies the surplus generated power to a generator to operate the generator when the regenerated power acquired by the regenerative braking is in excess of an allowable charge level of the battery, and thereby forcibly runs an engine to run an engine brake by using the power generated by the regenerative braking.
The device disclosed in the above publication operates the generator to obtain engine brake when the generated power exceeds the allowable charge level of the battery.
The battery is not charged until the charge level becomes full regardless of the supplied power if the charged level has not yet reached the full charge. The chargeable power varies according to the charge level of the battery.
Therefore, when the regenerated power exceeds the chargeable power, the battery can only be charged with the chargeable power even if the battery has an extra chargeable capacity in view of its charging level. On the other hand, the braking force in the regenerative braking is not determined for the purpose of charging the battery, but is determined according to a deceleration required by a driver. The regenerated power, which is actually generated by the motor, is directly influenced by a deceleration torque corresponding to the deceleration. The larger the deceleration torque is, the more the regenerated power is.
Therefore, if the generator is operated according to the charge level of the battery to obtain engine brake, it is determined that the battery has an extra chargeable capacity in view of the charge level on condition that the regenerated power is not greater than the allowable charge level of the battery, even when the regenerated power is in excess of the chargeable power when the driver wants to suddenly brake the vehicle. Therefore, the generator is not operated and the engine brake is not obtained. This decreases the power generated by the motor, and increases the number of times at which the friction brake is used. This accelerates the wear of the friction material, and may deteriorate the durability of the friction brake.